The wild, nearly out of control growth of smartphones and
tablets has computer makers rethinking what goes into their desktop operating systems.

Notably, Microsoft and Apple are taking significant measures to address the success of mobile on their next desktop operating systems by making them closer to their smaller counterparts.

So how do these early "fusion" OSes hold up?

Let's start with Apple's new version of OS X, which made its debut earlier today in the form of a preview to developers. Apple's calling it Mountain Lion, and as the name would suggest, it's not a stern-to-stem overhaul, as much as some refining and additions to what the company did with Lion last year.

Apple's core idea with Mountain Lion is to take some of the built-in software people have been using on their iPhones and iPads and make it available on the
Mac, all the while keeping everything in sync behind the scenes. Where Lion took steps to borrow the look and feel of certain mobile apps, Mountain Lion just takes some of the bigger set pieces from iOS outright.

There are likely to be changes made to Mountain Lion before it ships. But what we do know is that there are at least 10 big additions coming with OS X 10.8. Most of them are features that people have already been using on Apple's other big platform, and they focus on unifying communication between the two otherwise disparate operating systems.

One of the clearest examples of that is the new Messages software, which will replace the decade-old iChat. It's compatible with iOS 5's iMessage feature, meaning users on both platforms can chat with one another regardless of what device they're on.

Apple's also carried over its storage and syncing service iCloud, surfacing it in Finder and making it easier to access files shared with iCloud-enabled iOS apps. A new Notification Center pulls app and program notifications together in under one banner not unlike Growl. There's also a Game Center to port iOS-style gaming to the Mac, new sharing features for iOS-style social sharing of Tweets, photos, and Web pages, and AirPlay Mirroring, which will broadcast whatever's on your Mac to an
Apple TV.

Apple has not taken the Mac security threats from the past year lying down, either. The new Gatekeeper feature is designed to stop malware by controlling what programs can be installed, which lets you set a source-based reputation level for installed programs.

Notably absent from Mountain Lion is Siri, Apple's voice assistant feature that made its debut last October. This is curious, given that some of the OS X-supported iOS apps like Reminders and Notes work quite well with the voice-enabled controls on iPhones. Siri is not expected to join OS X before the launch of Mountain Lion, but Apple could surprise. There are also a handful of other iOS features that are missing in action for now.

Windows 8
Where Apple is working to convince people that the best of iOS can be migrated to Mac, Microsoft is looking to convince people that Windows can be reborn with Windows 8.

This creates a new way for people to interact with the operating system. For instance, instead of the traditional icon requiring a double-click to open a box with open, close, and minimize buttons in the corner, the Metro interface relies on a mobile-app style approach. Tap one of the tiles to open it, swipe to get into a previously-opened program, and--although this hasn't been confirmed--you'll probably have to use the Windows button or some similar mechanism to return to the Start screen. Tiles themselves are icons on steroids. They're large, and they can surface data from the app in real time.

Much more will be revealed about Windows 8 at the Mobile World Congress in two weeks in Barcelona, Spain. Now, Windows Phone appears to be a fine reason to have a major presence at MWC, but it wouldn't be a difficult leap to expect some serious Windows 8 desktop news at the show, such as how the nascent operating system will interact with Windows Phone devices.

Whatever the specifics of the announcements are, we can expect Windows 8 to have close ties to Windows Phone, which uses a nearly identical interface. Businesses are seriously considering the potential of Windows 8, and analysts expect the rising Windows Phone to help raise moribund companies like Nokia. Microsoft knows that it hasn't been the best life-partner, especially when you've been tempted by the iPad and iPhone, and so it's creating a unified desktop and mobile experience to romance you once more.

Cynics have been equating Windows 8 with a Hail Mary longshot, a last-ditch attempt to save a dying OS. Optimists prefer to look at it as if the always-slow-to-react Microsoft is simply finally getting into the mobile game in a big way, by leveraging its desktop customer base.

Meanwhile, Apple's problems have less to do with saving a languishing OS, versus making it more relevant and approachable to the millions who have purchased an iPhone or iPad in recent years, but not one of the company's computers. Until we learn more about how Windows 8 will integrate with Windows Phone, it would be foolish to claim that anybody besides Apple is leading the field.

Senior writer Seth Rosenblatt covers Google and security for CNET News, with occasional forays into tech and pop culture. Formerly a CNET Reviews senior editor for software, he has written about nearly every category of software and app available.